Metal materials, such as pure metals and metal alloys, for example, are typically used as solders in many electronic device packaging and other electronic manufacturing applications. The emission of alpha particles from certain isotopes may lead to single-event upsets (“SEUs”), often referred to as soft errors or soft error upsets. Alpha particle emission (also referred to as alpha flux) can cause damage to packaged electronic devices, and more particularly, can cause soft error upsets and even electronic device failure in certain cases. Concerns regarding potential alpha particle emission heighten as electronic device sizes are reduced and alpha particle emitting materials are located in closer proximity to potentially sensitive locations.
Uranium and thorium are well known as principal radioactive elements often present in metal materials which may radioactively decay according to known decay chains to form alpha particle emitting isotopes. Of particular concern in non-lead materials is the presence of polonium-210 (210Po), which is considered to be the primary alpha particle emitter responsible for soft error upsets. Lead-210 (210Pb) is a decay daughter of uranium-238 (238U), has a half-life of 22.3 years, and β-decays to bismuth-210 (210Bi). However, due to the very short 5.01 day half-life of 210Bi, such isotope is essentially a transient intermediary which rapidly decays to 210Po. The 210Po has a 138.4 day half-life and decays to the stable lead-206 (206Pb) by emission of a 5.304 MeV alpha particle. It is the latter step of the 210Pb decay chain, namely, the decay of 210Po to 206Pb with release of an alpha particle that is of most concern in metal materials used in electronic device applications.
The root cause of alpha emission in refined, high purity metals and alloys is 210Pb at concentrations that may be lower than parts per trillion (ppt), for example, remaining after processing. Current refinement processes have focused on removing bulk Pb to below 1 part per million (ppm) and with it a proportional fraction of 210Pb. However, if the 210Pb concentration is high enough, current refining processes will not remove it to low enough levels, even after multiple refinement passes.
Although 210Po and/or 210Pb may be at least in part removed by melting and/or refining techniques, such isotopes may remain as impurities in a metal material even after melting or refining. Removal of 210Po from a metal material results in a temporary decrease in alpha particle emissions from the metal material. However, it has been observed that alpha particle emissions, though initially lowered, will typically increase over time to potentially unacceptable levels as the secular equilibrium of the 210Pb decay profile is gradually restored based on any 210Pb remaining in the metal material. As used herein, the term “secular equilibrium” refers to a condition in which the quantity or concentration of a radioactive isotope within a material remains constant because its production rate is equal to its decay rate. For example, the concentration of a radionuclide B in a given sample is at secular equilibrium when the production rate of radionuclide B from the decay of a parent radionuclide A, is the same as the decay rate of radionuclide B into its daughter radionuclide C.